The present invention relates to a vacuum fluorescent display with phosphors that emit light upon bombardment of electrons.
Conventionally, a vacuum fluorescent display of this type has an envelope constituting a display surface which has at least partly light transmission properties. A phosphor layer is formed by applying phosphors to the inner side of the display surface of this envelope. A cathode electrode for emitting electrons toward the phosphor layer, a grid electrode for extracting the electrons from the cathode electrode, and an anode electrode for accelerating the electrons extracted from the cathode electrode are arranged in the vacuum space in the envelope. The electrons extracted from the cathode electrode by the grid electrode are accelerated by the anode electrode and bombard against the phosphor layer. Hence, the phosphors that form the phosphor layer are excited by the impact of the electrons, and the phosphor layer emits light in a color corresponding to the excited phosphor. The light emitted from the phosphor layer emerges from the display surface through an optical film.
At this time, if a high voltage is applied to the anode electrode, the electrons accelerated to a high speed bombard against the phosphor layer to increase the luminance. When the electrons bombard, however, the temperature of the phosphor layer increases sharply, and the luminance decreases sharply. Since the bombarding electrons have a high energy, the amount of X-rays generated by the phosphor layer increases, and the X-ray undesirably leak to the outside from the display surface.
In order to suppress this sharp temperature increase and generation of the X-rays of the phosphor layer, conventionally, the voltage to be applied to the anode electrode is suppressed low. When the voltage to be applied to the anode decreases, however, high-luminance light cannot be obtained.
In some projection type cathode ray tube, a countermeasure for preventing leakage of X-rays with a thick vacuum envelope and a countermeasure for preventing overheat of the phosphor layer with a liquid cooling mechanism are taken separately. In this case, the cathode ray tube becomes large in size and weight, so it cannot be employed in a small-size vacuum fluorescent display. In order to prevent X-ray leakage, the display surface (phosphor screen plate) of the envelope may be made of lead glass. In this case, however, a blackening phenomenon is caused by heat. Thus, the display surface of the envelope cannot be made of lead glass.